The present invention relates to the monitoring of volume to a patient during anesthesia, and more particularly, to a system that allows the monitoring of the tidal volume to the patient by the use of flow sensors that are located at the machine or distal end of the patient breathing circuit.
In general, anesthesia systems are utilized in operating rooms and comprise various equipment necessary to anesthetize the patient and maintain the patient in that state until the operation is completed and it is possible to terminate the introduction of the anesthetic agent. Such systems include various pressure regulators, flow control devices, gas mixing devices and vaporizers to provide an anesthetic laden stream of gas to the patient. The patient is connected to the system by means of a face mask or other device and which interfaces with the anesthesia system by means of a patient breathing circuit. The system provides the anesthetic laden gas to the patient during an inspiratory cycle and receives the patient's exhaled gases during an expiratory cycle.
With current anesthesia machines, it is important to monitor the tidal volume of such gas administered to that patient. The present systems provide one or more flow sensors that monitor flow at various times through the patient breathing circuit and then integrate that flow with respect to a specific time to arrive at the tidal volume. In many such systems, the flow sensor that monitors the flow is located in the expiration side of the breathing circuit and the expiration cycle time is used to determine the tidal volume.
With the standard circle system breathing circuit, the flow sensor or flow sensors are conveniently located at the distal end of the patient breathing circuit in the inspiratory or expiratory limbs, that is, the end of the patient breathing circuit that is affixed to the anesthesia machine. Such position is convenient and is possible due to the path of flow within the circle breathing system since it is basically a closed system and the flow of gas follows the direction of a circle with fresh gas added to the system.
As used herein, the distal end of a patent breathing circuit is considered to be the end of the patient breathing circuit that is attached to the anesthesia machine and the proximal end of the patient breathing circuit is considered to be that end that is attached to the patient administration device, that is the face mask, endotracheal tube or the like.
A problem arises, however, with the use of a Bain breathing circuit or Mapleson D circuit where a large central tube connects the anesthesia machine with the patient and there is a continual to-fro pattern of flow within that tube. As such, the patient basically rebreathes some of the exhaled gas from the large central tube while receiving fresh gas through a separate tube that either lies coaxially within the large central tube as in the Bain circuit or through a separate tube that is outside but parallels the large central tube (Mapleson D circuit), both of which introduce the fresh gas to the system at or near the proximal end of the large central tube. Accordingly, if the tidal volume is measured at the distal end of such circuit, the reading will be inaccurate since the fresh gas flow continues, even during expiration and the flow sensor in the expiratory gas stream will read that additional fresh gas flow administered during the expiratory cycle and thus will read a flow of gas that is not actually administered to the patient.
In such cases, the flow sensor must, therefore, be positioned at the proximal end of the patient breathing circuit, that is, at the end of the circuit that is connected to the patient so that it reads only the gases actually exhaled by the patient. The difficulty with such arrangement is that such circuits are commonly used for infants and the positioning of the flow sensor at the end of the patient circuit connected to the patient adds additional and undesirable dead space to the circuit and further adds additional bulk to a device that is affixed to the patient.
Further, the location of the flow sensor adjacent the patient also introduces additional connections into the patient breathing circuit at a point where accidental disconnects frequently occur. Each additional connection at the proximal end of the circuit increases the possibility of leakage or disconnection.
As further problem of having the flow sensor at the patient connection is the additional clutter to the overall patient breathing circuit and the access to the patient. Often, the Bain circuit is used for facial surgery since the single tube reduces the clutter and enhances the access to the patient's face. By adding the proximal sensor with the added tubing, wiring and the like the additional clutter makes facial operations that much more difficult to perform.
Accordingly, it would be advantageous to carry out the monitoring of the tidal volume of the patient at the distal end of the patient circuit even when the circuit is of the type that is of the Bain and Mapleson D circuit type with a large tube to the patient with the fresh gas provided by a separate tubing to that patient.